The diagnostic method of exfoliative cytology has been applied to lesions in the esophagus, stomach, duodenum, pancreas, bile ducts, and colon. The present study concerns the frequency with which the diagnosis of cancer, confirmed by subsequent findings, was anticipated by microscopic examination of cells obtained by lavage. The most difficult area was the duodenum, but malignant cells were recognized in 33 out of 55 cases in which cancer was primary or metastatic in the duodenum or existed in the pancreas or biliary system. In 95% of a large number of cases of cancer of the esophagus, stomach, and colon the results of exfoliative cytology were positive. False-positive results were infrequent with reference to esophagus, stomach, and duodenum and did not occur with reference to the colon. In 38 proved cases of cancer of the colon, cytological diagnosis was positive in 36 instances.